Aerobic biological systems are the most commonly used processes for treating wastewaters laden with biodegradable contaminants. Despite the prevalence of these systems, they utilize considerable amounts of energy for aeration, which translates to a high cost for adequate contaminant removal. This expense has spurred interest in using anaerobic biological systems for wastewater treatment. Anaerobic systems significantly reduce the energy demand required for treatment and also produce an energy source in the form of biogas. The biogas produced can be used to offset the energy demand for water conveyance and other system processes. Despite the benefits of utilizing anaerobic systems, technological hurdles still exist that have prevented their widespread application. Several of these hurdles are described below.
Domestic wastewater is typically of very low strength because it typically comprises black water (fecal matter and flush water) and yellow water (urine and flush water) that are mixed with large amounts of greywater (shower, sink, and laundry water). The strength of wastewater is typically represented by the amount of organic matter present, measured by the concentration of chemical oxygen demand (COD). Anaerobic processes are more efficient at treating higher strength wastewater due to increased microbial activity and increased contact between the microbial mass and the contaminants. Although dilute wastewaters can be treated anaerobically, to do so typically requires large reactors to manage the high volumes of water entering the system. Large reactors require higher capital investments for construction and maintenance, which can negate some of the benefits of choosing anaerobic over aerobic biological systems. Beyond domestic wastewaters, there are many industrial wastewaters that also fall into the category of dilute wastewaters.
Source separation has been suggested as a possible remedy for preventing the dilution of higher strength wastewaters. While source separation can, in theory, be utilized to prevent dilution, it is often costly to implement as it requires retrofitting existing infrastructure and the use of redundant conveyance systems. Source separation also requires user behavior change and buy-in to be effective. These two issues have prevented source separation from being widely used. Until a technological solution is available, dilute wastewaters will likely continue to be treated using aerobic biological systems despite their high cost.
Depending on their source, wastewaters can contain high levels of nitrogenous compounds. When nitrogen-rich wastewaters are introduced to anaerobic processes, the nitrogen is generally not removed and passes through the system as ammonium. High ammonia levels can inhibit methanogenesis, a key process in the creation of biogas, thus decreasing the amount of energy that can be recovered from wastewater treatment. High levels of ammonia can also cause scaling and struvite precipitation, which often result in severe problems with the pumps and tubing of anaerobic systems. This problem can be worse at building-scale treatment plants where less dilution is available. For these reasons, anaerobic systems are often not selected when treating nitrogen-rich wastewaters.
Energy is often required to heat either the feed or bioreactors used in anaerobic wastewater treatment. Such heating increases the inactivation rate of pathogens, especially helminths such as Ascaris, which are prevalent in developing countries. To be effective, the disinfection temperature and contact time should be at least 60° C. for 30 minutes. Heating is also effective at accelerating the disintegration and hydrolysis of particles, which abound in wastewater. Further, anaerobic processes excel at thermophilic (55° C.) conditions for hydrolysis and acid production, and mesophilic (35° C.) conditions for methanogenesis. Unfortunately, heating large volumes of wastewater and/or large reactors comes at a tremendous energy cost. For industrial wastewater or sludge (more concentrated organics and lower volume), there is often enough chemical energy contained with the wastewater to offset heating requirements. However, the situation is much more challenging for dilute wastewater. For example, the energy contained within municipal wastewater may be insufficient to provide enough biogas to offset the system's heating requirements. The problem is exacerbated in cold climates where the heating demand is highest. Although solar thermal energy can be integrated into the process to provide heating, it is not always an option in low-solar areas or during cloudy periods. Solar energy storage is a possibility but presents its own challenges.
With the rise in popularity of anaerobic wastewater treatment, concerns have been raised about fugitive methane emissions associated with uncaptured dissolved methane in the effluent. The problem is worse under psychrophilic conditions, due to higher solubility of methane, and for municipal wastewater, due to greater effluent volume for holding the dissolved methane. Although this problem can be alleviated through temperature increase and volume reduction, this would come at the price of higher energy costs.
From the above discussion, it can be appreciated that the majority of problems related to using anaerobic systems for wastewater treatment relate to the dilute nature of the wastewater. While many inventions have focused on improving the ability of these systems to handle low-strength wastewaters, they have not been able to solve the problem.